According to the manufacturing method of a molding die of a plastic optical element which is conventionally generally conducted, a blank (primary product) is manufactured by, for example, steel or stainless steel, and an alloy of an amorphous-like nickel and phosphorous is filmed in the thickness of about 100 μm by a chemical plating, which is called electroless plating, on it, and this plating layer is cutting processed by a diamond tool in an ultra-precise processing machine, and a highly accurate optical surface transfer surface for molding the optical surface of the optical element is obtained.
According to such a method of the conventional technology, because shapes of parts are created basically by the mechanical processing, the part accuracy is easily increased to near the movement accuracy of the processing machine. However, on the other hand, problems are generated in which: the mechanical processing and chemical plating processing are mixed in the manufacturing process and it is vexatiously complicated, and a long period of delivery is necessary; the manufacturing of blank (primary processed goods) is necessary, considering the thickness of plating layer; the plating processing is not always stable, and due to a deviation of composition of the blank or a soiled condition, the attaching strength of the plating layer is dispersed, or a pinhole-like defect which is called a pit, is generated; and because the creation of the optical surface transfer surface is necessary in the thickness of plating layer, there is a case where there is not any margin in the plating thickness when the optical surface transfer surface is re-worked, and the processing can not be conducted.
Further, according to the conventional technology, it is necessary that the optical surface transfer surface is largely cutting processed by a diamond tool, however, in such a case, the influence such as a condition of a cutting edge of the tool or processing condition, a change of processing environmental temperature is exerted, and there is also a problem that the shape of the optical surface transfer surface finished after cutting processing, is delicately dispersed. This processing dispersion of the optical surface transfer surface is due to the poorness of the machinability of the raw material. In general, the optical surface shape error of about 100 nm is generated, and even in the case where processing is conducted very carefully, the shape error of about 50 nm is remained. This is a limit of processing accuracy when a large amount of optical surface transfer surfaces of the same shape are created.
Further, recently, an optical element in which a ring-shaped zone diffraction groove (diffraction ring-shaped zone) is provided on the optical surface and by which the chromatic aberration is efficiently corrected, is put into practical use in the optical information recording field, and a large amount of optical elements are manufactured. As an optical material, plastic or glass is used, however, in the infrared optical system, a crystal material such as ZnSe is also used. Such an optical element can be produced in a large amount and effectively by molding, however, it is a very important problem, at the time of molding, how high-accurately and effectively minute diffraction grooves on the optical surface of the optical element are manufactured by the die for optical element molding.
For example, when minute patterns having the optical function such as the diffraction grooves are created on the optical surface transfer surface of the die for optical element molding, the sharpness of the cutting edge controls the exactness of the diffraction groove shape, and when it is transferred as the optical surface of the optical element, the diffraction efficiency is largely influenced.
Accordingly, for the purpose that the diffraction efficiency of the diffraction ring-shaped zone is not lowered, it is necessary that the dimension of the cutting edge is made sufficiently small, in such a case, because, on the small cutting edge portion, the cutting resistance is concentrically imposed, it is necessary that the incised amount is decreased, and the number of time of processing is increased until the entire optical surface is uniformly cut and removed. Further, it is necessary that the feed speed of the tool is made slow in order also to prevent the deterioration of the surface roughness of the optical surface by the cutter mark of the small cutting edge, and the optical surface transfer surface processing time of one time also becomes long. As a result, in the cutting processing of the die for molding of the optical element having the diffraction groove, because the cutting length is increased, wearing of the cutting edge of the tool is increased, and the tool change becomes often. That is, when the optical surface transfer surface having minute shape is processed by the conventional diamond cutting, because the life of the tool is very shortened and a time period for processing one optical surface transfer surface is also increased, it is necessary that the tool is changed often, therefore, the processing efficiency is very much lowered, and the productivity of the die for optical element molding is lowered, resulting in the rapid increase of the cost. Therefore, particularly, when the optical surface transfer surface having minute shapes on the surface is finished by the diamond cutting, the die manufacturing method which does not include the electroless nickel plating process, and which is simple, and whose delivery date is short, is desirable.
In addition to that, recently, it is tried that the minute structure which is several times smaller or less than the wavelength of the light source to be used, is provided on the optical surface and a new optical function is added to the optical element. For example, the ordinary light converging function by the refraction of the molded lens and a positive dispersion generated as the side reaction at the time are cancelled by using a large negative dispersion by the diffraction obtained when the diffraction groove is provided on the surface of the aspheric surface optical surface, and a method in which the achromatic function, which is originally impossible only by the refraction, is added to a single lens optical element, is put in practice in the objective lens for pick-up apparatus for optical disk, which is DVD/CD compatible. This uses the diffraction action by the diffraction groove whose dimension is several-ten times larger than the wavelength of the light transmitting the optical element, and an area by which the diffraction action by the structure sufficiently larger than the wavelength is managed in this manner is called a scalar area.
On the one hand, it is well known that, in a minute interval which is one several-th of the wavelength of the light transmitting the optical element, protrusions of the conical shape are formed under the crowded condition on the surface of the optical surface, thereby, the reflection suppress function of the light can be exhibited. That is, the refractive index change on the border surface between the air and the optical surface when the light wave is incident on the optical element, is not instantly changed from 1 to the refractive index of the medium as in the conventional optical element, but, it is gently changed by the conical shape of protrusions arranged in minute interval, thereby, the reflection of the light can be suppressed. The optical surface on which protrusions like this are formed is a minute structure which is so-called a moth eye, and when the structural bodies which are minuter than the wavelength of the light are arranged in a period shorter than the wavelength, any longer, each structure is not diffracted and acts on the light wave as an average refractive index. Such an area is generally called an equivalent refractive index area. Relating to such an equivalent refractive index area, for example, it is written in Institute of electronic information communication papers C Vol. J83-C No. 3 pp. 173-181, March, 2000.
According to the minute structure of the equivalent refractive index area, a larger reflection prevention effect can be obtained than the conventional reflection prevention coat while the angle dependency of the reflection prevention effect or wavelength dependency is deceased. However, according to plastic molding, because the optical surface and the minute structure are simultaneously created, it is considered that a merit in the production in which the lens function and the reflection prevention function are simultaneously obtained, and the after processing that the reflection prevention coat processing is conducted after the molding is not necessary, is large, and is remarked. Furthermore, when such a minute structure of an equivalent refractive index area is arranged in such a manner that it has the directionality to the optical surface, the strong optical anisotropy can be given to the optical surface. Therefore, the double refraction optical element, which is conventionally manufactured by cutting the crystal such as quartz crystal, can be obtained by molding. Further, when it is combined with a refraction or reflection optical element, a new optical function can be added to it. The optical anisotropy in this case is called a structural double refraction.
There is a resonance area in which the diffraction efficiency is rapidly changed by a slight difference of the incident condition, between the above-described scalar area and the equivalent refractive index area. For example, when the groove width of the diffraction ring-shaped zone is brought to narrow, a phenomenon (anomary) that the diffraction efficiency is rapidly decreased or increased at about several-times of the wavelength, is generated. When the chracteristic of this area is used, a wave-guide mode resonance lattice filter by which only a specific wavelength is reflected, is realized by the minute structure, and the same effect as an ordinary interference filter can be realized by a smaller angle dependency.
Hereupon, when the optical element is formed by using the scalar area, equivalent refractive index area or resonance area, it is necessary to form the minute protrusions (or hollows) on the optical surface. In order to make mass-production of the optical element having such minute protrusions (or hollows), generally, it can be said appropriate that the molding is conducted by making plastic as a raw material. However, in such a case, it is necessary that the optical surface transfer surface provided with the hollows (or protrusions) corresponding to the minute protrusions (or hollows) is provided in the die for molding of the optical element.
However, relating to the protrusion (or recess) of the equivalent refraction area or resonance area as described above, because it is necessary that protrusions (or hollows) are formed at the interval of several-tens or several-hundreds nm, it is very difficult by the mechanical processing including the cutting processing. In contrast to that, in Tokkaihei No. 2003-160343 (Patent Document 1), the die for optical element molding in which the amorphous alloy having the super-cooling liquid phase is adhered to the substrate, and the optical surface transfer surface for molding the optical surface of the optical element is formed in the amorphous alloy, is disclosed. On the optical surface transfer surface, corresponding hollows or protrusions are formed so that a plurality of protrusions or hollows are transfer-formed on the optical surface of the optical element molded by such a die for optical element molding.
However, as a problem which can be generated when the die written in Patent Document 1 is manufactured, the deformation of the master-die caused in the case of the sticking of the metallic glass to the master-die, or breaking and peeling of the metallic glass film layer generated thereby, or, in the case where single-striking is generated, or the press-pressure is too strong, is presumed. Particularly, when the minute structure such as diffraction groove or step difference shape is provided on the master-transfer surface of the master-die, there is also a possibility that breaking of the minute structure is generated.
Further, when the minute structure is blaze-shape or sinking comb-shape, it is difficult to push the metallic glass to the bottom of groove, as the result, there is also a possibility that the tip of the minute structure corresponding to the blaze-shape or sinking comb-shape transferred on the molding transfer surface of the die for optical element molding is brought to a rounded shape. When the optical element is molded by such a die for optical element molding, because its molding transfer surface shape is transferred onto the optical surface of the optical element as it is, the optical surface of the optical element in which the shape of the primary part of the blaze-shape or sinking comb-shape is inaccurate, is formed, and characteristic of the optical element is lowered.